COMEBACK TRAIL: Jason Isringhausen, who is trying to earn a job in the Mets' bullpen, has impressed pitching coach Dan Warthen, who said he has seen a lot more out of the right-hander than he expected. Photo: Anthony J. Causi

PORT ST. LUCIE — Jason Isringhausen’s trip down memory lane finished the day after the Mets officially invited him to spring training. He’s all business lately, and starting to turn a few heads at the team’s spring training complex.

You can maybe even remove the “long shot” label from the 38-year-old Isringhausen in his attempt to win a job in the Mets’ bullpen. It comes 12 years after the Mets traded their former “Generation K” prodigy.

“I’ve seen a lot more than I expected at any time,” pitching coach Dan Warthen said yesterday. “The ball is coming out of his hand great. He still has the Izzy curveball, and he’s added a nice little cutter and changeup. I couldn’t be more pleased.”

It’s early in camp, but Warthen said he’s stopped to consider the possibility the Mets have struck gold.

“It makes me think a lot,” Warthen said. “If Izzy can come in and continue to do exactly what he’s doing right now, he is a major part of this.”

Isringhausen will soon get his first chance to impress in the exhibition season. The right-hander recently incurred his first “dead arm” period of the spring, and yesterday began a three-day shutdown to recover.

But for the first time since undergoing Tommy John surgery 18 months ago, Isringhausen said he feels strong. His last notable season came in 2007, when he saved 32 games for the Cardinals.

“My main goal is just to stay healthy,” Isringhausen said. “Just make sure nothing falls off in camp and go from there. If I pitch well in spring, other teams will see it, so if there’s not a spot here maybe I’ll move on. But I think there are enough spots here where if I can impress them enough I’ll be fine.”

Isringhausen said he probably tried to rush back too fast last season after surgery. He appeared in seven games for the Reds’ Triple-A affiliate, posting a 9.53 ERA, and was jobless after the season. As a favor, Mets special adviser J.P. Ricciardi let Isringhausen audition 12 days ago, and was impressed enough to get the right-hander a minor league contract.

In Isringhausen, the Mets have a tie to Bobby Valentine’s era managing the team.

“[Valentine] was very smart — sometimes too smart,” Isringhausen said. “Kind of like Tony La Russa. Those guys are just so smart, they sometimes outthink the game too much, but you’re never going to get something by them. They are so far ahead of everybody else, it seems like.”

Isringhausen said La Russa is the best manager for whom he has played. It was under La Russa that Isringhausen saved a career-high 47 games for the Cardinals in 2004.

Isringhausen isn’t attempting a comeback for the money — he’s earned north of $56 million over his career — just a few more baseball thrills.

“I love the competition,” he said. “There’s a lot of the stuff that goes along with the game I don’t enjoy, all the politics, the business part of it. But I enjoy the 1-on-1 competition with a hitter, and it’s hard for me to find that anywhere else. I’m just trying to compete with these kids, and I’m having fun doing it.”

There is even a chance he will regain his old No. 44 with the Mets. Isringhausen, who is wearing No. 45 in camp, recently was offered his old number by Jason Bay. But Isringhausen isn’t going to jinx it.

“Let’s wait until the last week of spring,” he said. “Then we’ll talk about it.”